1. Field of the Invention
The present invention relates to a control pin for controlling the flow of liquid metal in a casting process. In particular, but not exclusively, it relates to a control pin for controlling the flow of nonferrous liquid metals such as aluminium and zinc.
2. Description of the Related Art
A typical metal casting process is described in U.S. Pat. No. 3,111,732. In that process, liquid metal is poured through a spout (or “underpour outlet”) into a mould, where the metal freezes to form a billet or slab. The flow of metal through the spout is controlled by a control pin (or “flow regulator”) that is located within the spout. The control pin may be raised to increase the rate of flow of metal through the spout, or lowered to decrease or interrupt the flow of metal.
Control pins are generally made of a refractory material, which is able to withstand the high temperature of the molten metal. The material must also be hard so as to resist wear on the end of the rod, where it presses against the seat in the spout. One of the most commonly used materials is dense fused silica (DFS). This material is quite tough and has good thermal shock characteristics, but silica is wetted and attacked by liquid aluminium and control pins made of this material therefore have to be provided with a non-stick protective coating, for example of boron nitride. This coating has to be reapplied frequently (for example every one or two pouring operations) and such pins therefore have a high maintenance requirement.
Further, although DFS is quite tough, it is susceptible to cracking and these cracks tend to propagate through the material during use. This can eventually cause part of the control pin to break away and block the pouring spout. As a precaution against this, a stainless steel wire is sometimes embedded in the DFS material to ensure that even if the control pin breaks, the broken part can still be withdrawn from the spout.
Another disadvantage with control pins made of DFS is that they tend to have a high heat capacity and have to be pre-heated prior to commencement of the metal pouring operation, to bring them up to or close to the temperature of the molten metal. This adds considerably to the complexity of the pouring operation and gives rise to the risk of a serious accident when transferring the hot control pin from the pre-heating oven to the spout. If the control pin is not pre-heated, the molten metal can solidify upon contact with the control pin, thus blocking the spout.
Other materials are sometimes used for the control pin including, for example, cement-based refractories. Such materials are not wetted by the aluminium and therefore suffer less damage and require less maintenance. However, they are fragile and are easily chipped or broken. Further, such pins have a high heat capacity and therefore need pre-heating.
It is also known to make control pins from graphite. However, graphite suffers from oxidation and erosion at the air-metal interface, which limits the useful life of the control pins made from this material. Also, like control pins made of DFS or cement-based refractories, graphite pins have a high heat capacity and so require pre-heating.
Another refractory material described in U.S. Pat. No. 5,880,046 comprises an aqueous solution of phosphoric acid with a mixture of wollastonite and colloidal silica. The material is said to have good thermal insulation characteristics and very good behaviour with respect to molten aluminium. However, it is quite soft and therefore not very hard-wearing.